Fix up the readme

[goodboy]

As per @nicoddemus's suggestion It'd be good to have an introductory example and a little more excitement there ;)

[hpk42]

let me randomly note here that pluggy is used successfully also by tox and by devpi-server and probably by others (i never checked). So i suggest to not emphasize "pytest plugin system" too much but rather "pluggy: a little field-proven plugin manager" or something.

[goodboy]

@hpk42 good point and will do :)